1. Field of the Invention
The invention relates generally to directional drilling navigation of boreholes in coal seams, and more specifically to radars and control systems for keeping the borehole drilling to the vertical center of the coal deposit.
2. Description of the Prior Art
The production of coal and methane depends upon the environment of the original coal bed deposit, and any subsequent alterations. During burial of the peat-coal swamp, sedimentation formed the sealing mudstone/shale layer overlying the coal bed. In deltaic deposits, high-energy paleochannels meandered from the main river channel. Oftentimes, the channels scoured through the sealing layer and into the coal seam.
High porosity sandstone channels often fill with water. Under the paleochannel scour cut bank, water flows into the face and butt cleats of the coal bed. Subsequent alterations of the seam by differential compaction cause the dip, called a roll, to occur in the coal bed. Faults are pathways for water flow into the coal bed.
Drilling into the coal bed underlying a paleochannel and subsequent fracking can enable significant flows of water to enter. The current state of the art in horizontal drilling uses gamma sensors in a measurements-while-drilling (MWD) navigation subsystem to determine when the drill approaches a sedimentary boundary rock. But if sandstone is protruding into the coal, such as results from ancient river bed cutting and filling, then the gamma sensor will not help. Sandstone does not have significant gamma emissions, so this type of detection is unreliable. Drilling within the seam cannot be maintained when the seam is not bounded by sealing rock.
Methane diffusion into a de-gas hole improves whenever the drillhole keeps to the vertical center of the coal seam. It also improves when the drillhole is near a dry paleochannel. Current horizontal drilling technology can be improved by geologic sensing and controlling of the drilling horizon in a coal seam.
One present inventor, Larry G. Stolarczyk, has described methods and equipment for imaging coal formations in geologic structures in many United States Patents. Some of those Patents are listed in Table I, and are incorporated herein by reference.
The present inventor, Larry Stolarczyk, describes the measuring of the thickness of ground deposit layers with a microstrip antenna, in U.S. Pat. No. 5,072,172, issued Dec. 10, 1991. Interpolation tables are used to lookup the layer thickness values corresponding to antenna conductance and resonance measurements. Such resonant microstrip patch antenna (RMPA) and their resulting measurements are used to guide coal-seam drum-cutter equipment for more efficient mining of natural deposit ores. The RMPA driving-point impedance (S11) changes significantly when a solid, gas, or liquid layer thickness overlying the RMPA varies.
U.S. Pat. No. 5,769,503, issued Jun. 23, 1998, to Stolarczyk, et al., describes mounting such RMPA on a rotating drum or arm of a coal, trona, or potash mining machine. A ground-penetrating-radar transmitting antenna and a receiving antenna can be mounted on a cutting drum to detect deeply buried objects and anomalous geology just ahead of the mining. A radar frequency downconverter is used so low-cost yet-accurate measurement electronics can be built. A first phase-locked loop (PLL) is operated at the resonant frequency of the patch antenna or at each sequentially stepped radar frequency. A second PLL is offset from the first PLL by an intermediate frequency (IF) and is called a tracking PLL. The measurement speed can be delayed by the sequential way in which the PLL""s lock on to signals, so a solution to that delay is described.
The calibration curves represent an analytical function that has been reconstructed from a set of discrete I and Q data points measured at each height (H). The discrete sensor height calibration data can be used to construct two different polynomials with the independent variable being the physical layer thickness or height (H). The physical height (H) is independently measured with acoustic height measurement electronics during the calibration process or by other means, such as an inclinometer on the boom of a mining machine. The two calibration polynomials are,
I(H)=Re H=bnHn+bnxe2x88x921Hnxe2x88x921+ . . . +b1H+boxe2x80x83xe2x80x83(1A) 
and
Q(H)=Im H=anHn+anxe2x88x921Hnxe2x88x921+ . . . +a1H+aoxe2x80x83xe2x80x83(1B) 
U.S. Pat. No. 5,325,095, describes a modulator that sequentially creates in-phase (I) and quadrature phase (Q) shifts in a frequency source signal. The frequency source signal is sequentially shifted by 0xc2x0 or 180xc2x0 (in-phase), then by 90xc2x0 or 270xc2x0 (quadrature) in passing through the phase modulator to the radar transmit antenna. The electronic circuits employ isolators. Isolators and quadrature modulator transmitters are costly and difficult to build with wide bandwidth. The receiver section of the radar receives the reflected signals from the target and uses a single frequency conversion design to transpose the received radar signal frequency to a lower frequency range where the I and Q signal measurements are sequentially made at each frequency in the stepped-frequency radar method that has become one of the standard ground penetrating radar practices. The I and Q signals contain the antenna sensor information. As is well known in the art, the sensor information is processed in a Fourier transform to convert frequency domain information to time domain information. The time domain information is used to determine the time (to) for the signal energy to travel to and return back to the radar. By knowing the velocity (v) in a dielectric natural media such as coal   v  =      c                  ϵ        c            
where c is the speed of light, xcex5c is the relative dielectric constant of coal (about 6). The distance to the reflective target is   d  =            c              2        ⁢                              ϵ            c                          xe2x80x83                                            ⁢                  t        o            .      
The relative dielectric constant must therefore be known to accurately to determine distance.
The velocity formula is made more complex whenever the natural media layer is not coal, trona, or some other high-resistivity liquid or solid. The velocity of radio waves generally depends on the frequency and resistivity of the natural medium. It is therefore preferable to simultaneously measure the in-situ dielectric constant, e.g., when using radar to measure depths. Stepped-frequency radars have separate transmitting and receiving antennas, and are circularly polarized antennae. But printed circuit antennas radiate front and back. To counter this, U.S. Pat. No. 5,325,095, teaches the placement of radar-energy absorbing material on one side of the printed circuit board to reduce the back lobe.
The antenna pattern is directed only to one side of the printed circuit antenna. Such antennas are preferably oppositely polarized so that they can be operated in continuous wave (CW) mode and in close proximity to each other. The transmitter and receiver sections operate concurrently. The radar return signals from the target will typically be repolarized opposite to the transmitted signal. The reflected wave can thus be readily measured by the receiving antenna and associated electronics. But not all the reflected signals are oppositely polarized. An electromagnetic wave traveling in a first media and into a second media is reflected at the interface.
Electromagnetic wave reflection occurs at the interface of two different dielectric media, and the reflection coefficient can be expressed in Equation (2) as,                               Γ          =                                                    E                s                                            E                p                                      =                                                                                ϵ                    1                                                  -                                                      ϵ                    2                                                                                                                    ϵ                    2                                    +                                                            ϵ                      1                                                                                                          ;                              σ            ωϵ                    ⁢                      xe2x80x83                    ⁢                      "LeftSkeleton"            1                                              (        2        )            
where, Es is the reflected electric field component of the electromagnetic wave, a vector; Ep is the incident electric field component of the electromagnetic wave, a vector; xcex51 is the relative dielectric constant of the first media; xcex52 is the relative dielectric constant of the second media; "sgr" is the electrical conductivity of the media; and, xcfx89=2xcfx80f and f is the frequency of the EM wave.
Briefly, a drillstring radar embodiment of the present invention comprises a measurements-while-drilling instrument for mounting just behind the drill bit and downhole motor of a drill rod. The instrument includes a ground-penetrating radar system connected to upward-looking and downward-looking horn antennas. These are used to electronically probe the interface of a coal seam with its upper and lower boundary layers. A dielectric constant sensor is included to provide corrective data for the up and down distance measurements. Such measurements and data are radio communicated to the surface for tomographic processing and user display. The instrument also includes a navigation processor and drill bit steering controls. The radio communication uses the drillstring as a transmission line and F1/F2 repeaters can be placed along very long runs to maintain good instrument-to-surface communication. A docking mechanism associated with the instrument and its antenna array allows the instrument to be retrieved back inside the drillstring with a tether should the drill head become hopelessly jammed or locked into the earth.
An advantage of the present invention is that a drillstring radar is provided that helps keep boreholes in the middle of a coal bed.
Another advantage of the present invention is that a drillstring radar is provided that can guide directional drilling in real-time.
A further advantage of the present invention is better yeilds of method can be realized because the boreholes are restricted to the optimum middle ground within a coal bed layer.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.